I have a friend who shoots .410 shotshells in his .444 and bags grouse at very close ranges. So, as Mooncoon says it can be done.
You'd just need to put an over powder and or filler wad on top of your powder, then the shot, then an over shot wad. If you just want to try it out without buying/making special wads - just use newspaper or magazine paper.
If you really want to hunt upland game with a muzzleloader you'd be better off buying a smoothbore.